The yearly economic losses related to friction and abrasion are estimated to be about 2-7% of the GDP in developed countries including the United States and European countries. A report by the U.S. Department of Energy in 1999 indicated that by adopting various measures to reduce friction and abrasion, motor vehicles and transmission systems in the United Sates save 120 billion US dollars each year. One of those measures includes the application of lubricant compositions in motor vehicles and industrial equipments.
Modern lubricant compositions are widely used in various applications such as motor oils, transmission fluids, gear oils, power steering fluids, shock absorber fluids, brake fluids, hydraulic fluids and greases. The lubricant compositions can have various functions such as (1) controlling friction between surfaces of moving parts; (2) reducing wear of moving parts; (3) reducing corrosion of surfaces of moving parts, particularly metal surfaces; (4) damping mechanical shock in gears; and (5) forming a seal on the walls of engine cylinders. Each lubricant composition can contain a base oil and, depending on the application, a combination of additives or modifiers, such as viscosity index improvers, pour point depressants, dispersants, detergents, anti-wear agents, antioxidants, friction modifiers, rust inhibitors, corrosion inhibitors, demulsifiers and anti-foams.
In general, semi-crystalline random copolymers (with 60-70 wt % C2, 100-133 CH3/1000C's) have higher thickening efficiency and higher shear stability due to the lower levels of short chain branching when compared to an amorphous random copolymer (40-50 wt % C2, 166-200 CH3/1000C's). This suggests that a linear backbone with a minimal amount of side branching is required to achieve high thickening efficiency and shear stability for a polyolefin in an oil solution.
However at low temperature, the waxy components that are present in the oil (such as waxy paraffins) will make the oil stop flowing at a higher temperature than its cold use temperature (e.g. −15° C. for Exxon 100LP base oil). The freezing of oil at cold temperatures (−30 to −35° C.) can cause catastrophic engine failure due to oil pan starvation and filter clogging. To prevent this, pour point depressants can be added to modify the structure of the waxy oils so that, as the oil cools, the wax does not form a structure that would otherwise trap the rest of the oil and so prevent flow or block filters. Examples of pour point depressants include polyalkylacrylates, long chain alkyl phenols and phthalic-acid dialkylarylesters, ethylene-butadiene, alpha olefin copolymers with 6-24 carbon atoms (e.g. 1-hexane and 1-octadecane). The principle of oil wax modification is described, for example, in Ashbaugh, H. S.; Radulescu, A.; Prud'homme, R.; Schwahn, D.; Richeter, D.; Fetters, L.; Macromolecules, 35, 7044-7053 (2002); and, Klamann, D.; Lubricants and Related Products, Verlag Chemie, 1984 pp 185-203.
To be successful, an oil viscosity modifier must have compatibility in a wide range of oil base stocks(paraffinic, napthalenic, aromatic) and give a balance of performance over a wide range of conditions (shear and temperature).
Semi-crystalline random copolymers are difficult to formulate to obtain robust performance at low temperature (insufficient wax modification capability). Thus, amorphous copolymers are preferred as they are completely soluble in oil at low temperature and the action of the pour point depressant (already present in the formulated oil) is enough to modify the wax and maintain the pourability of the oil.
The viscosity index is commonly used as a measure of the rate of change of viscosity of a fluid with temperature. This temperature dependency is common to all fluids including base oils. In general, the higher the viscosity index, the smaller is the relative change in viscosity with temperature. The viscosity index (VI) improver or viscosity modifier is used to reduce the temperature dependency of the viscosity of the lubricant compositions so that the lubricant compositions can be used over a wide temperature range. In the other words, the VI improvers prevent the lubricant compositions from becoming too thin at a high temperature, e.g., hot summer temperatures, and too viscous at a low temperature, e.g., cold winter temperatures. Some known VI improvers include polymethacrylates, olefin copolymers, such as ethylene-propylene copolymers and ethylene-propylene diene-modified copolymers (EPDMs), and hydrogenated styrenic block copolymers such as styrene-ethylene/butylene-styrene copolymer (SEBS).
The hydrogenated styrenic block copolymers generally offer good thickening efficiency and excellent low temperature performance. However, these hydrogenated styrenic block copolymers are relatively expensive and have a limited useful life because of their low shear stability.
The olefin copolymers, such as amorphous ethylene-propylene copolymers, may offer good low temperature performance but poor thickening efficiency at high temperatures. The comonomer units of olefin copolymers can be distributed in a tapered manner. Generally, the tapered olefin copolymers, such as tapered ethylene-propylene copolymer, are excellent thickeners, have improved low temperature performance, and are able to avoid undesirable interactions with the base oils.
Although there are many VI improvers available in the market for formulating lubricant compositions, there is always a need for new VI improvers for lubricant compositions with improved properties and flexibilities. In particular, it is desirable that VI improvers lend low turbidity to a lubricant composition and lend a low pour point.